Method for loading a gas accumulator

ABSTRACT

A method for loading a gas accumulator, which contains at least one solid storage material, in a first step, introducing a gas that is to be stored into the gas accumulator that is partially filled up with the solid storage material, the storage material expands by absorbing the gas, and, in a last step, additional gas that is to be stored is introduced into the gas accumulator, so that the solid storage material is compacted together with the gas contained in it. Also, a use of the method.

FIELD OF THE INVENTION

The present invention relates to a method for loading a gas accumulator which contains at least one solid storage material.

BACKGROUND INFORMATION

Gas accumulators, which contain at least one solid storage material, are used for storing gaseous ammonia, for example. This may me used, for example, in selective catalytic NO_(x) reduction in the exhaust tract of an internal combustion engine.

At present, ammonia is stored in storage materials from which it is liberated by thermal desorption. Suitable storage substances that can be used are, for example, salts, especially chlorides and/or sulfates of one or more alkaline earth elements and/or one or more 3d group elements, such as manganese, iron, cobalt, nickel, copper and/or zinc. Organic absorbents are also suitable ammonia storage substances which are used for storing ammonia in devices for catalytic NO_(x) reduction. Such accumulators are described in German Patent No. DE 197 28 343, for example.

Gas storage at present takes place in two steps. In systems based on metal salts as ammonia carriers, the storage substance, which is generally present as a powder, first has applied to it gaseous ammonia under pressure, in a pressure-resistant reaction vessel. The storage substance and the ammonia form a complex, in this context. The material created continues to be present as a powder. In a second step, the loaded powder is pressed to form tablets. During the course of this, the volume generally decreases to a quarter of the initial volume. This production process has the disadvantage, however, that it takes a lot of time based on the two-step procedure, and is therefore costly. In addition, because of the great volume decrease during pressing, only cord-shaped formations are able to be implemented, without bulges and undercuts. An adaptation of the pressed storage substance to the inner geometries of gas accumulators is not possible.

SUMMARY OF THE INVENTION

The method for loading a gas accumulator, according to the present invention, which contains at least one solid storage material, includes the following steps:

(a) introduction of a gas to be stored into the gas accumulator that is partially filled with the solid storage material,

(b) expansion of the storage material by the absorption of the gas,

(c) further introduction of the gas to be stored into the gas accumulator, so that the solid storage material becomes denser with the gas contained in it.

Because of the filling and compacting of the storage material within the gas accumulator, the expanded and compacted storage material adapts to the inner structure of the gas accumulator. This enables the utilization of the full storage volume. Bulges and undercuts are also able to be filled up in this way, using the solid storage material that contains the gas to be stored.

Because of the complete filling out of the gas accumulator with the compacted solid storage material, sensors which extend into the gas accumulator are also enclosed, for instance, by the compacted solid storage material. Such sensors are temperature sensors or gas sensors, for example. Using the sensors, one is able to determine the temperature and the gas content in the storage medium. This is particularly interesting if the stored gas will be discharged from the gas accumulator again.

The gas accumulator, which is loaded with gas that is to be stored, is preferably a container that is able to be closed in a pressure-resistant manner. A line that is connected to the container in a gastight manner opens out into the container. During loading of the gas accumulator, the gas to be stored is introduced into the container via the line, and when the gas is taken out, the gas is discharged again via the same line.

The solid storage material is generally present as a powder. The advantage of the powdery solid storage material is that, during expansion, it is able to expand in any direction. Thus, it is additionally not required that the absorption of the gas take place directionally. Even in the case of any nondirectional absorption of the gas by the solid storage material, a uniform expansion in the gas accumulator follows. In addition, during the expansion, a powdery charge spreads out into gaps and undercuts that are present. Parts extending into the accumulator are enclosed by the charge without being damaged.

Because of the absorption of the gas, first of all a uniform expansion of the solid storage material takes place. Only when the solid storage material has expanded to the extent that the entire inner region of the gas accumulator has been filled up, does it begin to be compacted due to the introduction of additional gas that is to be stored.

The introduction of the gas, that is to be stored, into the gas accumulator, preferably takes place at a pressure in the range of 1 to 20 bar absolute. The gas that is to be stored is particularly preferably introduced into the gas accumulator at a pressure in the range of 3 to 5 bar absolute. The increased pressure makes it possible to fill up the gas accumulator more rapidly. In addition, the air contained in the gas accumulator is displaced by the introduction of the gas. Before the introduction of the gas that is to be stored into the gas accumulator, the accumulator is preferably evacuated. For this purpose, it is possible, for example, to connect a pump to the accumulator and to remove gas contained in the gas accumulator via the pump. Because of the underpressure, the gas to be stored is subsequently sucked into the gas accumulator via the supply line. As soon as the storage material has expanded to the extent that the entire gas accumulator has been filled up with solid storage material, normal pressure prevails in the accumulator, so that additional introduction of gas is now only possible by the application of an overpressure.

On account of the evacuation of the gas accumulator, foreign gas molecules are removed from the storage material, so that they are not able to hinder the loading process and the compaction process. Alternatively to the evacuation of the gas accumulator, it is also possible first to flush the gas accumulator using the gas that is to be stored, in order to remove air that is contained in the gas accumulator.

The quantity of the solid storage material in the gas accumulator is preferably sized in such a way that, when the gas accumulator is completely filled up with the gas that is to be stored, the maximum degree of compaction of the storage material has been reached. The quantity of solid storage material required may be calculated in a simple manner from the storage volume of the gas accumulator. The quantity is a function of the storage material used and the absorption capacity of the storage material, in this context. Filling up while using less storage material leads to the storage material not being compacted to the maximum degree, after the expansion. The quantity of absorbed gas in the gas accumulator is consequently less than for the case of filling up in which the solid storage material is compacted to its maximum degree of compaction. In the case where the gas accumulator is filled up using more solid storage material than required, if one wishes to achieve the filling up of the entire storage volume at complete compaction, this leads to the storage capacity of the solid storage material not being completely exhausted.

In particular, if the gas accumulator is being used for storing ammonia, the solid storage material contains at least one chloride and/or sulfate of an alkaline earth metal or of a 3d group element or an organic adsorbent. Suitable 3d group elements are manganese, iron, cobalt, nickel, copper and zinc, for example. The storage materials may be used in each case individually or as a mixture of at least two of the compounds named, in this context. Usually, however, only one storage material is used. In the storing of ammonia, magnesium chloride, calcium chloride or strontium chloride are particularly preferred as the solid storage material. However, besides the chlorides or sulfates of an alkaline earth metal or of a 3d group element or of an organic adsorbent, any other solid storage material for gases known to one skilled in the art is also suitable. It is possible to use other solid storage materials, particularly if a gas different from ammonia is to be stored in the gas accumulator. The suitability of the solid storage materials depends on the gas to be stored, in each case, and is known to one skilled in the art. In response to the storage of gas in the solid storage material, the volume of the storage material usually increases. In the storage of ammonia, in particular, the volume typically increases approximately four-fold of the initial volume. The increase is a function of the solid storage material selected, in this context. Now, if the quantity of the solid storage material is optimally adjusted to the volume of the gas accumulator, the absorption of gas by the solid storage material leads to the solid storage material's expansion, until it reaches the inner walls of the gas accumulator and completely fills up the gas accumulator. In the process, the solid storage material, generally present as a powder, also gets into bulges and behind undercuts in the geometry of the gas accumulator. Additional introduction of the gas to be stored, after the expansion, leads to the gas that is to be stored becoming additionally absorbed in the solid storage material. Because of this, the solid storage material becomes compacted.

In order to introduce the gas to be stored into the gas accumulator, the gas accumulator is usually tightly closed except for one supply line, and entrapped air molecules are pumped off to evacuate the gas accumulator. The gas to be stored is introduced into the gas accumulator through the supply line. As described above, the introduction of the gas takes place preferably under increased pressure compared to the environmental pressure, in this instance. The gas to be stored is stored in the solid storage material. When magnesium chloride is used, for example, a metal complex of the general formula Mg(NH₃)_(x)Cl₂ is formed. In this formula, x means a whole number <6. Because of the storage of the ammonia in the metal chloride, there is a change in the volume of the metal-amine complex being created. Ammonia molecules are intercalated into the salt structure until the metal complex thus created completely fills up the volume of the gas accumulator. As soon as the solid storage material, along with the gas stored in it, has completely filled up the volume of the container, additional gas to be stored is incorporated in the porous charge in such a way that ever greater regions of the charge come together to form a solid agglomerate. In order to prevent blocking by solid agglomerate, the mobility of the gas to be stored may be favorably influenced, for instance, by a change in the temperature, through targeted heating, for example. The adsorption and the diffusion of the gas to be stored is controlled by the change in temperature. The gas to be stored can thus also advance by penetrating into deeper layers, for instance, and the loading process is not shut down prematurely. When using magnesium chloride, into which ammonia is intercalated, the saturation of the magnesium chloride with ammonia comes about at 6 molecules of ammonia per magnesium (chloride) molecule. As soon as Mg(NH₃)₆Cl₂ is present in the entire gas accumulator, the maximum degree of compaction has been reached.

By filling the solid storage material into a gas accumulator and by subsequently intercalating the gas to be stored in the storage material on the inside of the gas accumulator, the solid storage material takes on the desired shape by expansion and compaction, namely the entire inner volume of the gas accumulator, taking into account actuators and sensors, for instance, that are included in the gas accumulator. In a manner different from that of the pressed tablets of solid storage material, known from the related art, that have the gas stored in them, using the method according to the present invention, one is able to achieve a complete filling up of the gas accumulator and, with that, a maximum storage quantity of gas that is to be stored.

The method according to the present invention is preferably applied, for example, to filling up a gas accumulator with ammonia for the selective catalytic NO_(x) reduction of exhaust gases. The exhaust gas may be, for instance, an oxygen-containing exhaust gas of an internal combustion engine.

The internal combustion engine, for whose exhaust-gas treatment the ammonia is to be used, may be any internal combustion engine known to one skilled in the art. Normally, the gas accumulator filled up, corresponding to the method according to the present invention, with ammonia, is used for the selective catalytic NO_(x) reduction of exhaust gases from self-igniting internal combustion engines in motor vehicles. In the selective catalytic NO_(x) reduction in the exhaust gas tract of the internal combustion engine, a conversion takes place of the nitrogen oxides with ammonia to nitrogen and water.

The ammonia required for the selective catalytic NO_(x) reduction in the exhaust gas tract of the internal combustion engine is released again from the gas accumulator by thermal desorption, for example.

If the method for loading a gas accumulator is used for a gas that is different from ammonia, then, as a function of the solid storage material used, the withdrawal of the stored gas also usually takes place by thermal desorption. Alternatively, however, it is also conceivable, for example, that a second gas is introduced and an exchange of gases takes place in the gas accumulator. The second gas to be introduced dissolves the gas contained in the gas accumulator from out of the solid storage material, in this context, so that it flows out of the gas accumulator.

However, thermal desorption of the gas for its discharge is preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 show four consecutively executed method steps for filling up a gas accumulator.

DETAILED DESCRIPTION

FIG. 1 shows a first method step for filling up a gas accumulator.

A gas accumulator 1 contains a solid storage material 3. Solid storage material 3 is a salt or an organic adsorbent. As was described above, suitable salts are chlorides and/or sulfates of alkaline earth metals or of3d group elements, for example. These solid storage materials 3 are particularly suitable for storing ammonia.

Only a part of gas accumulator 1 is filled up by solid storage material 3, before the intercalation of the gas that is to be stored. Gas accumulator 1 is able to assume any desired geometry, in this context. It is also particularly possible for gas accumulator 1 to include bulges and undercuts, for example. For the monitoring of the filling-up process and the discharge process of gas, gas accumulator 1 may, for instance, also include sensors that are not shown here which extend into the gas accumulator, for example. Such sensors are pressure sensors or temperature sensors, for example. Sensors by the use of which the gas composition or the gas content in gas accumulator 1 may be recorded may also be provided.

At the beginning of the filling-up process, gas accumulator 1 is preferably evacuated. Gas 7 that is to be stored is introduced into gas accumulator 1 via a supply line 5. Supply line 5 is preferably connected to gas accumulator 1 in a gastight manner, so that no gas 7 that is to be stored is able to escape from gas accumulator 1 via a leak.

Gas 7 that is to be stored, which is introduced into gas accumulator 1 via supply line 5, is absorbed by solid storage material 3. The volume of solid storage material 3 increases because of this. Since solid storage material 3 is present in the form of powder, because of the volume increase of 3 there is a uniform expansion of solid storage material 3. Because of this, undercuts and bulges in gas accumulator 1 are also filled out by solid storage material 3, which has already absorbed gas 7 that is to be stored. The volume increase of solid storage material 3, by the absorption of gas 7 that is to be stored, is shown in FIG. 2.

A further absorption of gas 7 that is to be stored by solid storage material 3 leads to an additional expansion of solid storage material 3, until the volume of gas accumulator 1 is completely filled up with solid storage material 3. This is illustrated in FIG. 3.

After gas accumulator 1 is completely filled up with gas 7 that is to be stored, further gas 7 that is to be stored is introduced via supply line 5 into gas accumulator 1. Solid storage material 3 is compacted until entire gas accumulator 1 is filled up by compacted solid storage material 9. Compacted solid storage material 9 preferably contains the maximum possible quantity of gas. An ideal filling up of gas accumulator 1 with gas 7 that is to be stored is achieved in this manner. The filling up of gas accumulator 1 with compacted solid storage material 9 is shown in exemplary fashion in FIG. 4.

The quantity of storage material 3 in gas accumulator 1 is preferably selected in such a way that storage material 3, after compaction, contains the maximum possible quantity of gas 7 that is to be stored, has been compacted to the smallest possible volume and fills up the entire volume of the gas accumulator, in this context. In this way, one is able to achieve the maximum storage capacity of the gas accumulator.

EXAMPLE

Magnesium chloride is used as the solid storage material for storing ammonia. Because of the intercalation of ammonia into the magnesium chloride, Mg(NH₃)₆Cl₂ forms. The density of Mg(NH₃)₆Cl₂ is given by 1252 kg m³. Consequently, for a storage volume of one liter, a mass of 1252 g Mg(NH₃)₆Cl₂ comes about. The proportion of magnesium chloride in the complex is about 48.2 wt. %. From this, we obtain a mass of magnesium chloride of 603.5 g. This quantity is filled into a gas accumulator having a content of one liter. When ammonia is applied to the chamber, the free space is filled little by little, until a powdery framework made of partially loaded metal-amine salt of the general formula Mg(NH₃)_(x)Cl₂ fills out the volume. In this formula, x means a whole number <6. As soon as the partially loaded metal-amine salt fills out the entire storage volume, additional ammonia molecules are fitted into this framework, until the target material Mg(NH₃)₆Cl₂ has been achieved.

By this procedure it is possible to implement the filling-up process in one step, and, on the other hand, even complex three-dimensional shapes may be implemented. It is particularly possible to fill up completely even complex geometries. 

1. A method for loading a gas accumulator, which contains at least one solid storage material, the method comprising: introducing a gas that is to be stored into the gas accumulator that is partially filled up with the solid storage material; expanding the storage material by an absorption of the gas; and further introducing the gas that is to be stored into the gas accumulator, so that the solid storage material is compacted with the gas contained in it.
 2. The method according to claim 1, wherein the solid storage material is present in the form of a powder.
 3. The method according to claim 1, wherein the gas that is to be stored is introduced into the gas accumulator at a pressure in a range of 1 to 20 bar absolute.
 4. The method according to claim 1, wherein a quantity of the solid storage material in the gas accumulator is sized in such a way that, when the gas accumulator is completely filled up with the gas that is to be stored, a maximum degree of compaction of the storage material has been reached.
 5. The method according to claim 1, wherein the solid storage material contains at least one chloride and/or sulfate of an alkaline earth metal or of a 3d group element or an organic adsorbent.
 6. The method according to claim 1, wherein the solid storage material is magnesium chloride, calcium chloride or strontium chloride.
 7. The method according to claim 1, wherein before an introduction of the gas that is to be stored, the gas accumulator is evacuated or flushed using the gas that is to be stored.
 8. The method according to claim 1, wherein an adsorption and a diffusion of the gas that is to be stored are controlled, during the loading of the gas accumulator, by changing a temperature.
 9. The method according to claim 1, wherein the gas that is to be stored in the gas accumulator is ammonia.
 10. The method according to claim 1, wherein the method is used for filling up a gas accumulator with ammonia for a selective catalytic NO_(x) reduction of exhaust gases.
 11. The method according to claim 10, wherein the exhaust gases include an oxygen-containing exhaust gas of an internal combustion engine. 